As the most general fiber-reinforced composite material, a glass fiber-reinforced resin has been known which is composed of a resin and glass fibers impregnated therewith. This glass fiber-reinforced resin is generally opaque. A method for obtaining a transparent glass fiber-reinforced resin has been disclosed in Patent Documents 1 and 2 in which the refractive index of glass fibers is allowed to coincide with the refractive index of a matrix resin.
The presence of some bacteria that produce cellulose fibers has been known. In Patent Documents 3 and 4, there have been disclosed molded materials having various shapes, such as a sheet, a fiber, and a three-dimensional body, which are formed from cellulose fibers (hereinafter referred to as “bacterial cellulose” in some cases) produced by bacteria.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-207234    Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-156279    Patent Document 3: Japanese Unexamined Patent Application Publication No. 62-36467    Patent Document 4: Japanese Unexamined Patent Application Publication No. 8-49188
The glass fiber-reinforced resins disclosed in Patent Documents 1 and 2 may become opaque in some cases depending on working conditions. Since the refractive index of a material has a temperature dependence, even when being transparent under certain temperature conditions, the glass fiber-reinforced resins disclosed in Patent Documents 1 and 2 are changed to semitransparent or opaque under conditions different from the above temperature conditions. The refractive index of each material has its own wavelength dependence; hence, even when the refractive index of fibers and that of a matrix resin are allowed to coincide with each other at a specific visible light wavelength, a region in which the refractive index is shifted may probably exist in the entire visible light region, and as a result, in the region described above, the transparency cannot be obtained.
Bacterial cellulose disclosed in Patent Documents 3 and 4 is composed of monofilaments having a fiber diameter of 4 nm, and this fiber diameter is considerably smaller than a visible light wavelength, so that refraction of visible light is unlikely to occur. However, in Patent Documents 3 and 4, when being used to form a composite together with a resin, the bacterial cellulose is disaggregated for the use. When a product produced by bacteria is disaggregated by applying a mechanical shearing force by a grinder or the like, bacterial cellulose fibers are closely attached to each other during a disaggregation process to form bundles having a large fiber diameter which may cause refraction and scattering of visible light, resulting in degradation in transparency of a composite material formed using the disaggregated cellulose as described above.
As described above, as of today, there has not been provided a fiber-reinforced composite material that always maintains superior transparency regardless of temperature conditions and wavelength bands.
In a touch switch, there are generally provided a fixed contact support plate composed of a transparent base material and a transparent conductive film (fixed contact) formed on one surface of the base material and a traveling contact support plate composed of a transparent base material and a transparent conductive film (traveling contact) formed on one surface of the base material. The transparent conductive films of the two support plates are disposed to face each other with a spacer provided therebetween. When being pressed, the traveling contact support plate is curved, and the transparent conductive film which is the traveling contact of the traveling contact support plate and the transparent conductive film which is the fixed contact of the fixed contact support plate are brought into contact with each other.
In general, the base material of the fixed contact support plate is a glass plate or a transparent and insulating sheet of an acrylic resin, polycarbonate resin, polyester resin, or the like, having a thickness of approximately 75 μm to 5 mm. The base material of the traveling contact support plate is a transparent and insulating film or sheet having a thickness of approximately 75 to 200 μm, which is formed using a material similar to that mentioned above. On the base materials described above, transparent conductive films functioning as a contact, circuit patterns, connector lead portions, and the like are formed.
A glass plate is heavy and has an inferior impact resistance.
Although being lighter in weight than a glass plate, a resin sheet has the following problems. The transparent conductive film, the circuit pattern, and the like forming the contact on the base material are formed by pattern etching, and in a process for this pattern etching, heat may be applied in some cases. In this case, when the difference in coefficient of linear thermal expansion between the base material and the transparent conductive film is large, by a stress generated at the interface therebetween, damages, such as cracking and peeling, may be done to the transparent conductive film, and as a result, the conductivity may be degraded in some cases by the damages done to the transparent conductive film. When the traveling contact support plate has insufficient flexural strength and/or flexural modulus, the support plate may be deformed or broken by a pressing force repeatedly applied to the plate in operation. Also in this case, the conductivity is degraded due to the degradation of the transparent conductive film.
Accordingly, development of a transparent base material on which a transparent conductive film is formed has been desired, the transparent base material having superior transparency, a light weight, and a small coefficient of linear thermal expansion, causing no damage done to the transparent conductive film due to the coefficient of linear thermal expansion different from that of the transparent conductive film, having high flexural strength and/or flexural modulus, and being capable of sufficiently withstanding a repeatedly applied pressing force.